A previous generation of printing machines and procedures has focused on mixed resolution. These systems most typically have employed about 24 pixels/mm (600 pixel dots per inch, or "dpi") in a carriage scan direction transverse to the printing medium and 12 pixels/mm (300 dpi) in the print-medium advance direction longitudinal to the printing medium--or 24 pixels/mm for black and 12 pixels/mm for chromatic colors, or relatively tall 12 mm (half-inch) pens for black ink and relatively short 8 mm (third-inch) pens for chromatic colors; or combinations of these and other operating-parameter mixtures.
These mixed-resolution systems have been of interest for obtaining effectively very high quality printing with a minimum of developmental delay. In the continuing highly competitive development of inkjet printer products, the mixed systems have served a very important role because of many difficult problems associated with attaining a full ultrahigh resolution--for example 24 pixel/mm pens, 12 mm tall, for all colorants in a color-printing system.
In the current generation of machines, interest has shifted to solving those many difficult problems. As will be seen, most of such difficulties have been recognized for many years, but tend to be aggravated in the ultrahigh-resolution environment.
(a) Throughput and cost--In a sense many problems flow from these two considerations, since essentially all the problems would evaporate if it did not matter how slow or expensive a printer was. In practice, marketplace pressures have made it crucially important that a printer be both competitively fast (even when printing in a "quality" mode) and competitively economical.
(b) Firing frequency--Thus for example high throughput in combination with high resolution pushes the capability of economical inkjet nozzles to fire at a high enough repetition rate. An inkjet pen tends to be most stable in operation, and to work best for error hiding, at a low firing frequency.
Horizontal resolution of 24 pixels/mm if printed all in a single pass, however, would require a rather high firing frequency--in fact, for current-day technology, roughly twice the highest frequency of reliable operation in an economical pen. This figure may be expected to change with refinements in pens.
(c) Banding and pattern artifacts--These spurious image elements are well known in lower-performance printers, but like other problems can be even more troublesome in the newer generation of devices. It is known, for example, that some banding effects can be reduced by printing highly staggered (i.e., overlapping) swaths--but also that doing so reduces overall throughput proportionately. (A different kind of visible banding, associated with hue shifts, will be discussed below.) Hence, again, high throughput tends to run counter to elimination of banding, and this conflict is aggravated by a requirement for printing at resolution that is twice as fine.
As to pattern defects, the design of dither arrays is a logical culprit and has previously received a great deal of attention in this regard, and may be considered highly refined. Yet heretofore some patterning persists in high-resolution images printed under conditions which should yield the best possible image quality.
Theory suggests that no further advantage can be obtained through dither redesign, and that solutions must be sought elsewhere. Discussion of printmasks in a following subsection of this document will take up this theme again.
Generally speaking, tools for investigating this area heretofore have been inadequate.
(d) Color shift--One important approach to maximizing throughput is to print bidirectionally. In a bidirectional-printing system the pens print while the carriage is traveling in each of its two directions--i.e., across the printing medium, and back.
This technique is well known and successful for printing in monochrome. Workers skilled in this field have recognized, however, that for printing in color a hue shift, or more precisely a color shift, arises as between printing in the two directions.
The reason is that pens are traditionally arranged, physically, on their carriage in a specific sequence. Therefore if two or more of the pens fire while the carriage is moving in one particular direction the different ink colors are laid down one on top of another in a corresponding order--and while the carriage is moving in the opposite direction, in the opposite order.
Usually the first inkdrop of two superposed drops tends to dominate the resulting perceived color, so that for example laying down magenta on top of cyan produces a blue which is biased toward the cyan; whereas printing cyan on top of magenta typically yields a blue which emphasizes magenta. If successive separate swaths--or separately visible color bands, subswaths--are printed while the pen is thus traveling in each of two directions, respectively, the successive swaths or subswaths therefore have noticeably different colors. Banding that results is often very conspicuous.
For this reason, previous artisans have striven to avoid printing of any superposition-formed secondary colors in more than one order, ever. Printers commercially available under the brand names Encad.RTM. and Laser-master.RTM., in particular, employ a tactic that employs brute force to avoid sequence changes: the pens are offset, with respect to the vertical direction, or in other words longitudinally along the printing medium.
They are offset by the full height of each nozzle array--posing, at the outset, significant problems of banding (see discussion following) as between colors. Furthermore, in consequence of the full-height-offset arrangement each of the trailing three pens must print over a color subswath formed in at least one previous scan--from one to three previous scans, depending upon which pen is under consideration.
This system advantageously maintains a fixed color sequence even in bidirectional printing. Use of full-height offset of the pens, however, makes a great sacrifice in other operating parameters. More specifically, the full-height staggered pens have a print zone that is four color bands (subswaths) tall.
Necessarily the overall product size in the direction of printing-medium advance is correspondingly greater, as are weight and cost. In addition the extended printzone is more awkward to manage in conjunction with a round (i.e. cylindrical) platen.
Furthermore in this system it is considerably more awkward to hold the printing medium consistently flat and without relative motion. Still further, the trailing pen is overprinting a pixel grid that has already been inked by three preceding pens, and in a heavy-color region of an image this means that a considerable amount of liquid has already been laid down on the page, and the page has had a significant time to deform in response.
Substantial and uncontrollable intercolor registration problems may be expected--particularly in view of the fact that this liquid-preloading effect is differential as between the several pens. In other words, it is present even for the second pen in the sequence, but suffered with progressively greater severity by the third and fourth.
The Encad/Lasermaster systems use bidirectional printing for at least the so-called "fast" and possibly "normal" printing modes, but not for the "best"-quality mode (which prints unidirectionally). Of course use of unidirectional printing as a best-quality printing mode incurs a throughput penalty of a factor as high as two. (Because the retrace may be at a faster, slew speed the factor may be less than two.) Such a penalty can be very significant.
Thus the art has failed to deal effectively with hue shifts--an impediment to fully exploiting the potential of bidirectional printing as a means of enhancing through-put.
(e) Liquid loading--Hue shift, however, is not the only problem that is associated with bidirectional printing. Another is microcoalescence. This may be regarded as a special case (particularly afflicting ultrahigh-resolution operation) of excessive inking with its historically known problems--which are summarized below.
In still another difficulty, the tails or satellites of secondary-color dots, pointing in opposite directions, can generate textural artifacts when the left-to-right order is reversed.
Excessive inking is a more-familiar problem. To achieve vivid colors in inkjet printing with aqueous inks, and to substantially fill the white space between addressable pixel locations, ample quantities of ink must be deposited. Doing so, however, requires subsequent removal of the water base--by evaporation (and, for some printing media, absorption)--and this drying step can be unduly time consuming.
In addition, if a large amount of ink is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise: so-called "bleed" of one color into another (particularly noticeable at color boundaries that should be sharp), "blocking" or offset of colorant in one printed image onto the back of an adjacent sheet with consequent sticking of the two sheets together (or of one sheet to pieces of the apparatus or to slipcovers used to protect the imaged sheet), and "cockle" or puckering of the printing medium. Various techniques are known for use together to moderate these adverse drying-time effects and bulk- or gross-colorant effects.
(f) Prior print-mode techniques--One useful and well-known technique is laying down in each pass of the pen only a fraction of the total ink required in each section of the image--so that any areas left white in each pass are filled in by one or more later passes. This tends to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and also may facilitate shortening of drying time.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a "printmode". Heretofore artisans in this field have progressively devised ways to further and further separate the inking in each pass.
Larry W. Lin, in U.S. Pat. No. 4,748,453--assigned to Xerox Corporation--taught use of a simple checkerboard pattern, which for its time was revolutionary in dividing inking for a single image region into two distinct complementary batches. Lin's system, however, maintains contact between pixels that are neighbors along diagonals and so fails to deal fully with the coalescence problem.
The above-mentioned U.S. Pat. No. 4,965,593, which is in the name of Mark S. Hickman, teaches printing with inkdrops that are separated in every direction--in each printing pass--by at least one blank pixel. The Hickman technique, however, accomplishes this by using a nozzle spacing and firing frequency that are multiples of the pixel-grid spacing in the vertical and horizontal directions (i. e., the medium-advance and scan axes respectively).
Accordingly Hickman's system is not capable of printing on intervening lines, or in intervening columns, between the spaced-apart inkdrops of his system. This limitation significantly hinders overall throughput, since the opportunity to print such further intervening information in each pass is lost.
Moreover the Hickman system is less versatile. It forfeits the ability to print in the intervening lines and columns even with respect to printmodes in which overinking or coalescence problems are absent--such as, for example, a high-quality single-pass mode for printing black and white text.
The above-mentioned U.S. Pat. No. 5,555,006, which is in the name of Lance Cleveland, teaches forming a printmask as plural diagonal lines that are well separated from one another. Cleveland introduces printmodes that employ plural such masks, so that (unlike Hickman) he is able to fill in between printed elements in a complementary way.
It is certainly not intended to call into question the Cleveland teaching, which represents a very substantial advance in the art--over both Lin and Hickman. Cleveland's invention, however, in part is aimed at a different set of problems and therefore naturally has only limited impact on general overinking problems discussed here. In particular Cleveland seeks to minimize the conspicuousness of heater-induced deformation at the end of a page.
Thus even Cleveland's system maintains the drawback of inkdrop coalescence along diagonals and sometimes--since he calls for very steeply angled diagonal lines which in some segments are formed by adjacent vertical pixels--even along columns.
Another ironic development along these lines is that the attempts to solve liquid-loading problems through printmask tactics in some cases contribute to pattern artifacts. It will be noted that all the printmodes discussed above--those of Lin, Hickman, Cleveland, and other workers not mentioned--are all highly systematic and thus repetitive.
For example, some printmodes such as square or rectangular checkerboard-like patterns tend to create objectionable moire effects when frequencies or harmonics generated within the patterns are close to the frequencies or harmonics of interacting subsystems. Such interfering frequencies may arise in dithering subsystems sometimes used to help control the paper advance or the pen speed.
(g) Known technology of printmodes--One particularly simple way to divide up a desired amount of ink into more than one pen pass is the checkerboard pattern already mentioned: every other pixel location is printed on one pass, and then the blanks are filled in on the next pass.
To avoid horizontal "banding" problems (and sometimes minimize the moire patterns) discussed above, a printmode may be constructed so that the printing medium is advanced between each initial-swath scan of the pen and the corresponding fill-swath scan or scans. This can be done in such a way that each pen scan functions in part as an initial-swath scan (for one portion of the printing medium) and in part as a fill-swath scan.
This technique tends to distribute rather than accumulate print-mechanism error which is impossible or expensive to reduce. The result is to minimize the conspicuousness of--or, in simpler terms, to hide--the error at minimal cost.
The pattern used in printing each nozzle section is known as the "printmode mask" or "printmask", or sometimes just "mask". The term "printmode" is more general, usually encompassing a description of a mask--or several masks, used in a repeated sequence or so-called "rotation"--and the number of passes required to reach full density, and also the number of drops per pixel defining what is meant by "full density".
Operating parameters can be selected in such a way that, in effect, mask rotation occurs even though the pen pattern is consistent over the whole pen array and is never changed between passes. Figuratively speaking this can be regarded as "automatic" rotation or simply "autorotation".
As mentioned above, some of these techniques do help to control the objectionable patterning that arises from the periodic character of printmasks employed heretofore. Nevertheless, for the current new generation of ultrahigh-resolution color printers generally speaking the standards of printing quality are higher, and a more-advanced control of this problem is called for.
(h) Further limitations--Heretofore color printing with at least partially swath-aligned discrete inkjet printheads has not been bidirectional. This limitation is very significant in terms of optimizing printing quality against throughput. In particular such color printing has not employed multipass printmodes for optimum bidirectional strategies.
Some of these characteristics are known, but only in conjunction with plural-color pens integrated into a unitary head. It has not heretofore been suggested to provide such features in a system that uses separate, discrete printheads. Indeed, myriad problems and obstacles would have stood in the way of applying that earlier knowledge to discrete-head systems--even if such a suggestion had been known heretofore.
(i) Conclusion--Thus persistent problems of firing frequency, hue shift, liquid loading, and pattern artifacts, counterbalanced against pervasive concerns of throughput and cost, have continued to impede achievement of uniformly excellent inkjet printing. It may be added that certain combinations of these difficulties are more readily controlled on one and another printing medium; however, at least some of these problems remain significant with respect to all industrially important printing media.
Thus, as can be seen, important aspects of the technology used in the field of the invention remain amenable to useful refinement.